1. Field of the Invention
This invention relates generally to static VAR generators, and more particularly to static VAR generators employing switched capacitors in conjunction with switched inductors for regulating an AC network.
2. Description of the Prior Art
The switching of capacitors into an AC network for power factor correction or voltage control must be accomplished with a minimum of disturbance to the system. To minimize high voltage disturbances on the AC network the capacitors must be switched to and from the system at current zero when the system voltage is at its positive or negative peak. Once the capacitors have been disconnected from the system they gradually discharge over a period of time. To maintain peak voltage on the capacitors they must be periodically switched back into the system. Recharging of the capacitors results in current surges and voltage transients in the AC network. In addition, maintaining the capacitor banks at full voltage subjects them to DC stress and requires more expensive DC-type capacitors. It has been proposed in the U.S. Pat. No. 3,703,680 issued to Frank et al. to switch the capacitor banks into the system for reversing the charge every half period in synchronism with the maximum of the network voltage. Since the charge on the capacitor banks can alternate with the network voltage it is proposed that the capacitor banks can be maintained at peak voltage without subjecting them to DC stress and, consequently, AC capacitors can be used. However, charging current surges and thus voltage transients in the network remain a problem.
It is desirable when regulating an AC network to be capable of responding to the needs of the system as quickly as possible. Because capacitors must be switched into the system when the system current is zero and the system voltage is either positive or negative, power system engineers would like to manage their system so that when the system demands capacitors they are available of both polarities to be switched in at the very next current zero crossover. Otherwise, if the system demands increased capacitance and if at the next current zero crossover the system voltage is positive and the capacitors are all negative charged, or vice versa, the system would have to wait another half cycle until the system voltage matches the capacitor voltage. To eliminate this problem, it would be desirable to have the capability to reverse the charge on capacitors when it is found that all the capacitors in the bank have the same polarity. However, engineers are reluctant to insert capacitors into the system for charge reversal due to the transient disturbances to the system that can result. It would be advantageous to those skilled in the art if there could be found a means of switching in capacitance without the effect of creating disturbances in the system. See also, U.S. Pat. No. 4,234,843 issued to L. Gyugyi which utilizes inductance with capacitance switching.